The expanding use of Internet services, electronic mail, file transfer and other home-office applications, continues to drive the need for higher bandwidth access to the home and small business operations. The Digital Subscriber Line (DSL) technology, including Asymmetrical Digital Subscriber Line (ADSL), has provided an effective mechanism to make use of existing copper access loops and to significantly increase transmission speeds while permitting parallel usage of telephone and Internet services.
The Discrete Multitone (DMT) modulation technique is a particularly suitable way of implementing DSL in order to provide high speed and efficient use of existing bandwidth. In DMT, multiple narrow bandwidth sub-carriers all transmit at once in parallel and each carry a small fraction of the total information. The sub-carriers or tones, each corresponding to a subchannel, have different center frequencies, are independently modulated wit data and are processed in parallel. For example, in DMT, the bandwidth may be divided between 256 sub-carriers, each with a 4 kHz bandwidth. Multi-carrier modulation requires orthogonality between all the sub-carriers and a Fast Fourier Transform (FFT) is a convenient method of achieving this modulation. At low frequencies, where copper wire attenuation is low and signal to noise ratio (SNR) is good, it is common to use a high bit rate, but at higher frequencies when unfavourable line conditions exist, modulation is reined to accommodate lower signal to noise ratio. Impulse noise which may be generated by electrical appliances, lightning or with a phone going off hook or ringing, is wideband in frequency and narrow in time so it is spread over many DMT subchannel and its influence over any subchannel is relatively small. Nonetheless, the line impulse response can result in symbol distortion at the receiver. In order to offset the line impulse response, it is known to insert a cyclic prefix (M) in the time domain samples. The cyclic prefix between symbols tends to reduce inter-symbol interference. Frequently, however, the line impulse response will last longer than the typical cyclic prefix time samples and, to address this, it is known to implement a time domain channel equalizer (TEQ) algorithm. The TEQ algorithm, which applies a Fast Fourier Transform, attempts to shorten the line impulse response into M time samples. However, the response can never be exactly the finite duration of the M samples regardless of the kind of TEQ algorithm implemented. As a result, there is always channel leakage outside the M samples which causes symbol distortion or inter-channel interference at the receiver. Although the interference level is mostly very small, the interference is not evenly distributed about the channels and some channels may experience much higher interference which will cause receiver data error.